1. Field of the Invention
The present invention generally relates to a catalytic purifying system for reducing nitrogen oxide (NOx) contained in an exhaust emission, and in particular to a technique enabling it for a computer-incorporated control unit to always discriminate that there is no remainder of a liquid reducing agent, there remains a different kind of liquid agent, or there is a liquid reducing agent that is normal by using a sensor unit capable of issuing an output signal in relation to the concentration of a liquid reducing agent on the basis of a characteristic of heat transmission between two points apart from each other within a storage tank capable of storing the liquid reducing agent.
2. Description of the Related Art
As a catalytic purification system for reducing nitrogen oxide (NOX) included in engine exhaust gas, Japanese Unexamined Patent Publication No. 2000-27627 discloses an exhaust gas purifying apparatus. In the exhaust gas purifying apparatus, a liquid reducing agent of a necessary amount depending on the engine operating conditions is injected into an exhaust gas moving on an upstream side of a reduction catalytic converter, which is disposed in an engine exhaust system to make a catalytic reduction reaction between NOX in the exhaust gas and the liquid reducing agent, thereby converting NOX to harmless components.
In the conventional exhaust gas purifying apparatus, however, when purification efficiency changes as the concentration of the liquid reducing agent changes, if the driver continues the operation of an engine without noticing it, required NOX reducing performance cannot be achieved, and there occurs such an undesirable potential that a large amount of NOX is inconveniently exhaust into the atmosphere. In particular, when there is no remainder of the liquid reducing agent or there remains a solution of a different kind which does not function as a liquid reducing agent is used, the above-mentioned inconvenient problem conspicuously appears.
Consequently, it has been contrived to provide such a sensor that is able to detect concentration of a liquid reducing agent on the basis of the characteristic of heat transmission between two points within the liquid reducing agent which are separated apart from each other. However, when the sensor is mounted on a mobile vehicle such as a car, the following problem might occur. Specifically, since the vehicle body is constantly subjected to vibrations due to the waviness of the road surfaces and the like while a mobile vehicle is running, convective motion of the liquid reducing agent is generated in a storage tank. When the convection is generated in the liquid reducing agent, the heat transmission characteristic of the liquid reducing agent workable as a heat transmission medium changes, and the concentration detection accuracy accordingly deteriorates quite a lot. Consequently, it becomes difficult to detect concentration of the liquid reducing agent during travel of the mobile vehicle, and it cannot be said that the sensor is sufficiently utilized.